High-Pressure Reactions on Metals in SBA-15
Ching-Yu Wang, Kai Shen, John M. Vohs, and Raymond J. Gorte
University of Pennsylvania
It was recently shown that deactivation due to coking can be greatly suppressed for ethene dimerization over Ni in mesoporous materials at pressures higher than that at which pore condensation occurs [Agirrezabal-Telleria & Iglesia, J. Catal. 352 (2017) 505]. However, preparation of catalysts in which all of the metal is in the mesopores can be difficult and silica may not be the ideal support for all reactions. In the present talk, we will describe simple methods for modifying the surface composition of SBA-15 and for incorporating various catalytic metals into the pores of SBA-15. We will also report preliminary results for using these catalysts at high pressures.
Vapor-phase infiltration of organometallic precursors was used to incorporate Pt, Pd, Rh, Ru, and Ni into SBA-15. The evacuated SBA-15 was exposed to a few torr of the precursor molecules between 443 and 523 K, resulting in monolayer films of the adsorbed precursors inside the mesopores. The metal particles that formed after removal of the precursor ligands remained in the pores and had particle sizes ranging from 3.8 nm for Pt to 5.2 nm for Ni, as determined by Transmission Electron Microscopy (TEM), XRD, and CO chemisorption. When alkane dehydrogenation reactions were performed over Pt/SBA-15 as a function of pressure, there was shown to be a critical pressure, above which the dehydrogenation reaction is stable in the absence of added H2. Although equilibrium yields were low at high pressures, the H2 produced by these reactions can be used in other reactions. Finally, we show that the surface composition of SBA-15 could be modified by depositing thin films of TiO2, WOx, ZrO2, and CaCO3 using Atomic Layer Deposition. Calorimetric measurements with hexane and benzene showed that these films significantly changed the hydrocarbon adsorption properties of the mesoporous material, a result which could allow tuning of the critical pressure.